This invention relates to a process for the production of an electrode grade petroleum coke having superior grindability qualities. More specifically, this invention is directed towards an improved delayed coking process which produces a soft, nonabrasive coke particularly suitable for use in the manufacture of carbon anodes used in the electrolytic production of aluminum.
Petroleum coke is the residue resulting from the thermal decomposition or pyrolysis of high-boiling hydrocarbons, particularly residues obtained from cracking or distillation of asphaltenic crude distillates. The hydrocarbons employed as feedstocks in coking operations usually have an initial boiling point of about 700.degree. F or higher, an API gravity of about 0.degree. to 20.degree., and a Conradson carbon residue content of about 5 to 40 weight percent. The coking process is particularly advantageous when applied to refractory, aromatic feedstocks such as slurry decanted oils from catalytic cracking and tars from thermal cracking.
Delayed coking is a process to increase yields of gas oil and gasoline which employs a heating means, a coking chamber designed to accumulate substantial quantities of coke between cleanings, and a fractionator which recovers valuable constituents from the volatiles driven off from the coking chamber. In a typical delayed coker, preheated coker feedstock is combined with heavy residue passing from the bottom of the fractionator and this mixture is heated in a tube still heater or furnace to a temperature of about 900.degree. F (generally within the range from 800.degree.-1100.degree. F). The heated mixture then passes to a coking drum where the residence time is sufficient for coke to form and settle from the mixture. The vapors from the coking drum are returned to the fractionator where gas, gasoline, and gas oil are separated and leave the unit. The heavier materials appear in the bottom of the fractionator and are recycled to the coking operation. When coke builds up to a predetermined level in one of the coking drums, flow is diverted to another drum so that the operation is semi-continuous. Thus, drums are operated in pairs with one on-stream while the other is being decoked.
The principal market for petroleum coke is the aluminum industry, which consumes coke in the form of carbon anodes employed in the electrolytic recovery of primary aluminum. Although the specifications for green and calcined coke set by the aluminum industry show some variations, they may be summarized as shown in Table 1 below.
Table 1 ______________________________________ Specifications of Petroleum Coke For Manufacture of Aluminum Anodes Green Calcined ______________________________________ Volatiles, percent max 12.0 0.5 Ash, percent max 0.15-1.0 0.05-0.8 Calcium, percent max 0.12 0.12 Iron, percent max 0.06 0.05-0.08 Silicon, percent max 0.06-0.08 0.05-0.08 Sodium, percent max 0.12 0.12 Combined Fe-Si, percent max 0.14 0.14 Combined Na-Ca, percent max 0.12-0.15 0.15 Soluble salts, percent max 0.2-0.8 0.8 Sulfur, percent max 1.5-2.0 1.0-2.0 Real Density 2.01-2.07 ______________________________________
A description of the processes for producing electrode grade coke from a residue obtained by the thermal cracking of certain hydrocarbon distillates obtained from asphaltic crude oils, followed by the delayed coking of the residue under controlled conditions is described in U.S. Pat. No. 2,775,549, issued to Shea. As disclosed therein, a coking stock suitable for the production of a premium coke is characterized by the absence of materials or components which would ordinarily produce inferior quality coke, such as asphaltic and napthenic base oils. Such materials have a tendency to coke at a much lower temperature and at a faster rate than the rest of the coker feed, resulting in premature and non-uniform coking. According to the patent, these materials should be removed from coker stocks prior to coking by a heat treating step or a solvent extraction step or both.
Other more recent patents have also disclosed methods of controlling the effects of premature coking. For example, U.S. Pat. No. 3,547,804 discloses a process wherein the delayed coker feed is diluted with a diluent which is not substantially cokable and heated to a temperature varying between 715.degree. to 770.degree. F before raising the mass to the coking temperature to complete the coking. The object of the method is to avoid the formation of amorphous coke about minute particles of massy coke resulting from premature coking, i.e., coking which occurs in the asphalt.fwdarw.pitch step of the production of coke from aromatic hydrocarbons. The diluent is employed to avoid amorphous coke formation during heating within the specified temperature range so that only crystal nuclei of acicular structure are formed during this period. Subsequently, when the mass is heated to the coking temperature, coke crystals are developed about these nuclei as centers.
U.S. Pat. No. 3,704,224 discloses the use of graphite seeding to improve coke quality. Fine colloidal graphite particles are added to the delayed coker feedstock as it passes from the furnace to the coke drum. Thus in this process, nuclei are added to the coker feedstock by seeding after the coker feedstock has been heated to the coking temperature; whereas, in the diluent process of U.S. Pat. No. 3,547,804 nuclei are formed in the feedstock before it is heated to the coking temperature.
A problem in applications requiring size reduction of the coke product which none of the foregoing disclosures address is the grindability of the resulting coke mass. To make either artificial graphite electrodes or amorphous carbon electrodes, coke must be ground to a very fine size before being calcined and admixed with a carbonaceous binder (usually coal tar) and charged to the electrode fabrication system. Therefore, a highly desirable quality of electrode-grade coke is high grindability. Some cokes are difficult to grind because they contain hard, dense, graphitic particles up to about one inch in diameter. Such coke particles may be referred to as "shot coke". The Hardgrove index of shot coke may range as low as 20 to 30 percent. A more desirable coke may be referred to as "sponge coke," which has a structure of porosity and is therefore a relatively easy-to-grind coke. The Hardgrove index of sponge coke is within the range from about 30 to 60 percent or higher. Accordingly, an improved delayed coking process which produces sponge coke conforming to the specifications set for green coke to be used in the preparation of carbon anodes (see Table 1) is needed.
A number of methods involving the concurrent processing of oil and coal or other carbonaceous materials have been suggested in the prior art. For example, U.S. Pat. No. 2,412,879 contains a description of a method of forming highly frangible, relatively soft coke by continuously coking a heavy petroleum oil in the presence of from 1 to 10 percent by weight of added cellulose linters, threads, sawdust, wood flour, or other cellulosic materials in divided form. More recently, processes for the treatment of coal and related materials with relatively low-boiling, highly aromatic oils to extract liquid constituents from the coal have been developed (see U.S. Pat. No. 3,870,621). However, none of these processes are directed toward the production of electrode grade coke; review of these and related disclosures has not revealed a coking process wherein the principal feedstream is a petroleum-derived coking stock and the product coke is a sponge coke advantageously employed in the manufacture of carbon electrodes.